Oximetry is the measurement of the oxygen level status of blood. Early detection of low blood oxygen level is critical in the medical field, for example in critical care and surgical applications, because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of oxygen supply. A pulse oximetry system generally consists of a sensor applied to a patient, a pulse oximeter, and a patient cable connecting the sensor and the pulse oximeter.
The pulse oximeter may be a standalone device or may be incorporated as a module or built-in portion of a multiparameter patient monitoring system, which also provides measurements such as blood pressure, respiratory rate and EKG. A pulse oximeter typically provides a numerical readout of the patient""s oxygen saturation, a numerical readout of pulse rate, and an audible indicator or xe2x80x9cbeepxe2x80x9d that occurs in response to each pulse. In addition, the pulse oximeter may display the patient""s plethysmograph, which provides a visual display of the patient""s pulse contour and pulse rate.
FIG. 1 illustrates a prior art pulse oximeter 100 and associated sensor 110. Conventionally, a pulse oximetry sensor 110 has LED emitters 112, typically one at a red wavelength and one at an infrared wavelength, and a photodiode detector 114. The sensor 110 is typically attached to an adult patient""s finger or an infant patient""s foot. For a finger, the sensor 110 is configured so that the emitters 112 project light through the fingernail and through the blood vessels and capillaries underneath. The LED emitters 112 are activated by drive signals 122 from the pulse oximeter 100. The detector 114 is positioned at the fingertip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues. The photodiode generated signal 124 is relayed by a cable to the pulse oximeter 100.
The pulse oximeter 100 determines oxygen saturation (SpO2) by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor 110. The pulse oximeter 100 contains a sensor interface 120, an SpO2 processor 130, an instrument manager 140, a display 150, an audible indicator (tone generator) 160 and a keypad 170. The sensor interface 120 provides LED drive current 122 which alternately activates the sensor red and IR LED emitters 112. The sensor interface 120 also has input circuitry for amplification and filtering of the signal 124 generated by the photodiode detector 114, which corresponds to the red and infrared light energy attenuated from transmission through the patient tissue site. The SpO2 processor 130 calculates a ratio of detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on that ratio. The instrument manager 140 provides hardware and software interfaces for managing the display 150, audible indicator 160 and keypad 170. The display 150 shows the computed oxygen status, as described above. The audible indicator 160 provides the pulse beep as well as alarms indicating desaturation events. The keypad 170 provides a user interface for such things as alarm thresholds, alarm enablement, and display options.
Computation of SpO2 relies on the differential light absorption of oxygenated hemoglobin, HbO2, and deoxygenated hemoglobin, Hb, to determine their respective concentrations in the arterial blood. Specifically, pulse oximetry measurements are made at red and IR wavelengths chosen such that deoxygenated hemoglobin absorbs more red light than oxygenated hemoglobin, and, conversely, oxygenated hemoglobin absorbs more infrared light than deoxygenated hemoglobin, for example 660 nm (red) and 905 nm (IR).
To distinguish between tissue absorption at the two wavelengths, the red and IR emitters 112 are provided drive current 122 so that only one is emitting light at a given time. For example, the emitters 112 may be cycled on and off alternately, in sequence, with each only active for a quarter cycle and with a quarter cycle separating the active times. This allows for separation of red and infrared signals and removal of ambient light levels by downstream signal processing. Because only a single detector 114 is used, it responds to both the red and infrared emitted light and generates a time-division-multiplexed (xe2x80x9cmodulatedxe2x80x9d) output signal 124. This modulated signal 124 is coupled to the input of the sensor interface 120.
In addition to the differential absorption of hemoglobin derivatives, pulse oximetry relies on the pulsatile nature of arterial blood to differentiate hemoglobin absorption from absorption of other constituents in the surrounding tissues. Light absorption between systole and diastole varies due to the blood volume change from the inflow and outflow of arterial blood at a peripheral tissue site. This tissue site might also comprise skin, muscle, bone, venous blood, fat, pigment, etc., each of which absorbs light. It is generally assumed that the background absorption due to these surrounding tissues is relatively invariant over short time periods and can be easily removed. Thus, blood oxygen saturation measurements are based upon a ratio of the time-varying or AC portion of the detected red and infrared signals with respect to the time-invariant or DC portion:
RD/IR=(RedAC/RedDC)/(IRAC/IRDC)
The desired SpO2 measurement is then computed from this ratio. The relationship between RD/IR and SpO2 is most accurately determined by statistical regression of experimental measurements obtained from human volunteers and calibrated measurements of oxygen saturation. In a pulse oximeter device, this empirical relationship can be stored as a xe2x80x9ccalibration curvexe2x80x9d in a read-only memory (ROM) look-up table so that SpO2 can be directly read-out of the memory in response to input RD/IR measurements.
Pulse oximetry is the standard-of-care in various hospital and emergency treatment environments. Demand has lead to pulse oximeters and sensors produced by a variety of manufacturers. Unfortunately, there is no standard for either performance by, or compatibility between, pulse oximeters or sensors. As a result, sensors made by one manufacturer are unlikely to work with pulse oximeters made by another manufacturer. Further, while conventional pulse oximeters and sensors are incapable of taking measurements on patients with poor peripheral circulation and are partially or fully disabled by motion artifact, advanced pulse oximeters and sensors manufactured by the assignee of the present invention are functional under these conditions. This presents a dilemma to hospitals and other caregivers wishing to upgrade their patient oxygenation monitoring capabilities. They are faced with either replacing all of their conventional pulse oximeters, including multiparameter patient monitoring systems, or working with potentially incompatible sensors and inferior pulse oximeters manufactured by various vendors for the pulse oximetry equipment in use at the installation.
Hospitals and other caregivers are also plagued by the difficulty of monitoring patients as they are transported from one setting to another. For example, a patient transported by ambulance to a hospital emergency room will likely be unmonitored during the transition from ambulance to the ER and require the removal and replacement of incompatible sensors in the ER. A similar problem is faced within a hospital as a patient is moved between surgery, ICU and recovery settings. Incompatibility and transport problems are exacerbated by the prevalence of expensive and non-portable multiparameter patient monitoring systems having pulse oximetry modules as one measurement parameter.
One aspect of the present invention is a dual-mode physiological measurement apparatus having a portable mode and an integrated mode. In the integrated mode, the measurement apparatus operates in conjunction with a multiparameter patient monitoring system (MPMS). In the portable mode, the measurement apparatus operates separately from the MPMS. The measurement apparatus has a physiological measurement processor, a display, a MPMS interface and a management processor. The physiological measurement processor has a sensor input and provides a physiological measurement output. In the portable mode, the display indicates a physiological parameter according to the physiological measurement output. In the integrated mode, the MPMS interface provides a communications link between the measurement apparatus and the MPMS. The management processor has as an input the physiological measurement output. The management processor controls the display in the portable mode and communicates the measurement output to the MPMS via the MPMS interface in the integrated mode.
In one embodiment, the measurement apparatus described in the previous paragraph further comprises a plug-in module. The plug-in module comprises the measurement processor and the MPMS interface and possibly the display and management processor and is configured to be removably retained by and electrically connected to the MPMS in the integrated mode. The plug-in module may further comprise a patient cable connector providing the sensor input, a keypad accepting user inputs in the portable mode, and a module connector mating with a corresponding MPMS backplane connector in the integrated mode. In another embodiment, the measurement apparatus further comprises a docking station and a portable portion. The docking station has a docking portion, a plug-in portion and the MPMS interface. The plug-in portion is configured to be removably retained by and electrically connected to the MPMS. The portable portion comprises the measurement processor, the display and the management processor. In the integrated mode, the portable portion is configured to be removably retained by and electrically connected to the docking portion. In the portable mode, the portable portion is separated from the docking station and operated as a standalone patient monitoring apparatus. The portable portion may further comprise a patient cable connector providing the sensor input, a keypad accepting user inputs in the portable mode, and a portable connector mating with a corresponding docking station connector in the integrated mode.
Another aspect of the present invention is a patient monitoring method utilizing a standalone measurement apparatus and a multiparameter patient monitoring system (MPMS) comprising the steps of performing a first physiological measurement with the standalone apparatus physically and electrically isolated from the MPMS and presenting information related to the first measurement on a display portion of the standalone apparatus. Further steps include performing a second physiological measurement with the standalone apparatus interfaced to the MPMS, communicating the second physiological measurement to the MPMS, and presenting information related to the second measurement on a monitor portion of the MPMS.
One embodiment of the patient monitoring method described in the previous paragraph further comprises the step of plugging the measurement apparatus into a module slot portion of the MPMS so that the measurement apparatus is in electrical communications with the MPMS. Another embodiment further comprises the steps of plugging a docking station into a module slot portion of the MPMS so that the docking station is in electrical communications with the MPMS, and attaching the standalone apparatus to the docking station so that the standalone apparatus is in electrical communications with the docking station.
Yet another aspect of the present invention is a physiological measurement apparatus comprising a sensor responsive to a physiological state, a measurement processor means for calculating a physiological parameter based upon the physiological state, which presents the physiological parameter to a person, a packaging means for housing the measurement processor and the display and for providing a connection between the sensor and the measurement processor means, and an interface means for electrically connecting the packaging means to a multiparameter patient monitoring system (MPMS) in an integrated mode and for disconnecting the packaging means from the MPMS in a portable mode. In a particular embodiment, the packaging means comprises a module means for plugging into a slot portion of the MPMS. In another particular embodiment, the physiological measurement apparatus further comprises a docking station means for plugging into a slot portion of the MPMS. In the integrated mode, the packaging means is configured to attach to the docking station.